Electric power generating apparatus and control method for electric power generating apparatus

ABSTRACT

When electric power is obtained from a thermoelectric element, a value of electric current is changed, and two combinations of the electric current value and a voltage value (i 1 , V 1 ) and (i 2  and V 2 ) are obtained. An electric current value i t  at which the maximum electric power Wmax can be obtained is obtained using an electric current-voltage characteristic estimated based on the obtained two combinations of the electric current value and the voltage value. The electric current is controlled such that the value of the electric current becomes equal to the obtained value i t  when the electric power is obtained from the thermoelectric element. The value i t  is represented by an equation, i t =(i 2 V 1 −i 1 V 2 )/ 2 (V 1 −V 2 ).

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2004-40407 filed onFeb. 17, 2004 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electric power generating apparatus using athermoelectric element. More particularly, the invention relates tocontrol of generated electric power in an electric power converter in anelectric power generating apparatus which is provided in an exhaustsystem of an automobile or the like, and which supplies electric powergenerated due to a temperature difference between exhaust gas and acooling medium, using the electric power converter.

2. Description of the Related Art

A thermoelectric power generating apparatus is known, which recovers, aselectric power, heat energy contained in exhaust gas discharged from acombustion device such as an engine of an automobile. In such athermoelectric power generating apparatus, one end of a thermoelectricelement is heated by exhaust gas, and the other end of thethermoelectric element is cooled by a cooling medium such as coolant,whereby electric power is generated in the thermoelectric element due toa temperature difference between the exhaust gas and the cooling medium.After the voltage of the generated electric power is increased andsmoothed by the electric power converter, the electric power is suppliedto a battery and the like.

Meanwhile, output electric power of the thermoelectric element variesdepending on an electric current value when the output electric power isobtained. Further, this electric current-electric power (voltage)characteristic varies also depending on a temperature at each of bothends of the thermoelectric element. Accordingly, as disclosed inJapanese Patent Application Publication No. JP-A-6-22572, in order toefficiently obtain electric power when a condition on a high-temperatureside of the thermoelectric element and a condition on a low-temperatureside of the thermoelectric element change, for example, when electricpower is generated using exhaust gas of an automobile, it is necessaryto control electric current when the electric power is obtainedaccording to the electric current-electric power (voltage)characteristic. In this technology, a temperature on thehigh-temperature side of the thermoelectric element and a temperature onthe low-temperature side of the thermoelectric element are measured; anelectric current value at which the output electric power becomesoptimal is obtained based on an output characteristic of thethermoelectric element at the measured temperatures, the outputcharacteristic being obtained in advance; and the electric current iscontrolled to become equal to the obtained electric current value whenthe electric power is obtained.

In order to closely attach temperature sensors on the high-temperatureside and the low temperature side of the thermoelectric element, afixing member such as a spacer is required. The fixing member and thetemperature sensors serve as thermal resistance. As a result, the amountof heat transmitted in the thermoelectric element is decreased, andelectric power generation efficiency is also reduced. Also, ordinarily,multiple thermoelectric elements are connected in order to increase theamount of generated electric power. However, since temperatures at bothends of the thermoelectric element vary with each thermoelectricelement, it is difficult to detect overall tendency in the temperaturesusing the small number of temperature sensors. Meanwhile, if the numberof the temperature sensors is increased, cost is increased, and theelectric power generation efficiency is reduced. Therefore, increasingthe number of the temperature sensors is also difficult. Also, in orderto accurately detect a transient change in the temperatures, it isnecessary to enhance responsiveness of the temperature sensors.Particularly when responsiveness of the temperature sensor on thehigh-temperature side is enhanced, the cost is increased.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an electricpower generating apparatus using a thermoelectric element, which makesit possible to efficiently obtain electric power without requiringtemperature measurement.

In order to achieve the aforementioned object, a first aspect of theinvention relates to an electric power generating apparatus including athermoelectric element; and an electric power converter which controlselectric current when the thermoelectric element generates electricpower. The electric power converter changes the electric current whenthe thermoelectric element generates the electric power so as to obtainvoltage values V₁ and V₂ corresponding to at least two differentelectric current values i₁ and i₂, the electric current value i₁ beingsmaller than the electric current value i₂; and the electric powerconverter controls the electric current such that a value of theelectric current becomes equal to an electric current value i_(t) whenthe thermoelectric element generates the electric power, the electriccurrent value i_(t) satisfying an equation, i_(t)=(i₂V₁−i₁V₂)/2(V₁−V₂).

An electric current (i)-voltage (V) characteristic of the thermoelectricelement varies depending on a temperature on a high-temperature side ofthe thermoelectric element and a temperature on a low-temperature sideof the thermoelectric element. Ordinarily, the electric current(i)-voltage (V) characteristic is represented by an equation, V=V₀−k×i(in this equation, 0≦i≦V₀/k. The values of V₀ and k vary depending onthe temperature on the high-temperature side of the thermoelectricelement and the temperature on the low-temperature side of thethermoelectric element. Refer to FIG. 3). Thus, output electric power Wis represented by an equation, W=i×(V₀−k×i)=i×V₀−k×i². Accordingly, themaximum electric power Wmax is obtained in the case of “dW/di=0”, thatis, in the case of “i_(t)=V₀/(2×k)”. In the case where combinations ofthe electric current value and the voltage value (i_(t), V₁) and (i₂,V₂) are obtained when the temperatures on the high-temperature side andthe low-temperature side of the thermoelectric element are giventemperatures, the electric current value i_(t) at which the maximumelectric power can be obtained is calculated according to an equation,i_(t)=(i₂V₁−i₁V₂)/2×(V₁−V₂).

In the first aspect of the invention, the electric power converter mayinclude an ammeter that measures a value of electric current flowing inthe thermoelectric element, and a voltmeter that measures a value ofvoltage between output terminals of the thermoelectric element.

In the first aspect of the invention, the thermoelectric element mayinclude a high-temperature side end surface and a low-temperature sideend surface, and generates the electric power according to a temperaturedifference between the high-temperature side end surface and thelow-temperature side end surface.

In the first aspect of the invention, the high-temperature side endsurface may be provided in a vicinity of an exhaust passage of anautomobile, and a cooling device is provided on the low-temperature sideend surface.

A second aspect of the invention relates to an electric power generatingapparatus including a thermoelectric element; and an electric powerconverter which controls electric current when the thermoelectricelement generates electric power. The electric power converter changesthe electric current when the thermoelectric element generates theelectric power to obtain electric power values W₁ and W₂ correspondingto at least two different electric current values i₁ and i₂, theelectric current value i₁ being smaller than the electric current valuei₂; the electric power converter estimates an electric current-electricpower characteristic based on a combination of the electric currentvalue i₁ and the obtained electric power value W₁ and a combination ofthe electric current value i₂ and the obtained electric power value W₂;the electric power converter obtains an electric current value at whichmaximum electric power can be obtained using the electriccurrent-electric power characteristic; and the electric power convertercontrols the electric current such that a value of the electric currentbecomes equal to the electric current value at which the maximumelectric power can be obtained when the thermoelectric element generatesthe electric power.

As described above, the electric current-voltage characteristic isrepresented by the equation, W=i×(V₀−k×i)=i×V₀−k×i². The electriccurrent-voltage characteristic is obtained based on the combination ofthe electric current i₁ and the obtained electric power value W₁ and thecombination of the electric current i₂ and the obtained electric powervalue W₂, and the electric current value i_(t) is obtained using thiselectric current-voltage characteristic. More specifically, the electriccurrent value i_(t) may be obtained after the values of k and V₀ areobtained, or the electric current value i_(t) may be stored in a storagedevice.

When plural electric current-electric power characteristics areestimated based on the combination of the electric current value i₁ andthe obtained electric power value W₁ and the combination of the electriccurrent value i₂ and the obtained electric power value W₂, the electricpower converter may obtain one of a voltage value and an electric powervalue corresponding to an electric current value i₃, the electriccurrent value i₃ being different from the electric current values i₁ andi₂, and the electric power converter may select the electriccurrent-electric power characteristic from among the estimated pluralelectric current-electric power characteristics based on a combinationof the electric current value i₃ and one of the voltage value and theelectric power value corresponding to the electric current value i₃.

Ordinarily, the electric current-electric power characteristic variesdepending on the two constant values V₀ and k. Therefore, when twocombinations of the electric current value and the voltage value areobtained, the values of V₀ and k should be determined, and the electriccurrent-electric power characteristic should be determined. However,since there is a limit to the accuracy of detecting the electric currentvalue and the electric power value, the accuracy cannot be made higherthan necessary. In the case where plural electric current-electric powercharacteristics are estimated when considering an error of each detectedvalue or a change in the accuracy, another combination of the electriccurrent value and the voltage value or the electric power value isobtained, and the electric current-electric power characteristic isselected from among the plural estimated electric current-electric powercharacteristics.

According to the invention, the electric current value and the voltagevalue or the electric power value are measured, and it is not necessaryto provide an additional sensor in the thermoelectric module includingthe thermoelectric element. Therefore, heat resistance is not changed,and electric power generation efficiency is not reduced. Also, it ispossible to accurately measure the electric current value, the voltagevalue, the electric power value, and the like using relatively low-costsensors. Also, the responsiveness of these sensors is good. Accordingly,it is possible to promptly obtain the optimal electric current value atwhich the maximum electric power can be obtained. Also, it is possibleto appropriately perform the electric current control even when there isa transient change in an engine operating state.

Also, these sensors can be included in the electric power converter. Inthis case, it is easy to assemble the electric power generatingapparatus, and the manufacturing cost thereof can be reduced.

In the second aspect of the invention, when plural electriccurrent-electric power characteristics are estimated based on thecombination of the electric current value i₁ and the obtained electricpower value W₁ and the combination of the electric current value i₂ andthe obtained electric power value W₂, the electric power converter mayobtain one of a voltage value and an electric power value correspondingto an electric current value i₃, the electric current value i₃ beingdifferent from the electric current values i₁ and i₂, and the electricpower converter may select the electric current-electric powercharacteristic from among the estimated plural electric current-electricpower characteristics based on a combination of the electric currentvalue i₃ and one of the voltage value and the electric power valuecorresponding to the electric current value i₃.

In the second aspect of the invention, the electric power converter mayinclude an ammeter that measures a value of electric current flowing inthe thermoelectric element, and a voltmeter that measures a value ofvoltage between output terminals of the thermoelectric element.

In the second aspect of the invention, the electric power converter mayinclude an ammeter that measures a value of electric current flowing inthe thermoelectric element, and a wattmeter that measures a value ofelectric power output from the thermoelectric element.

In the second aspect of the invention, the thermoelectric element mayinclude a high-temperature side end surface and a low-temperature sideend surface, and may generate the electric power according to atemperature difference between the high-temperature side end surface andthe low-temperature side end surface.

In the second aspect of the invention, the high-temperature side endsurface may be provided in a vicinity of an exhaust passage of anautomobile, and a cooling device may be provided on the low-temperatureside end surface.

A third aspect of the invention relates to a control method for anelectric power generating apparatus including a thermoelectric element;and an electric power converter which controls electric current when thethermoelectric element generates electric power. The control methodincludes the steps of changing the electric current when thethermoelectric element generates the electric power so as to obtainvoltage values V₁ and V₂ corresponding to at least two differentelectric current values i₁ and i₂, the electric current value i_(t)being smaller than the electric current value i₁; and controlling theelectric current such that a value of the electric current becomes equalto an electric-current value i_(t) when the thermoelectric elementgenerates the electric power, the electric current value i_(t)satisfying an equation, i_(t)=(i₂ V₁−i₁V₂)/2 (V₁−V₂).

A fourth aspect of the invention relates to a control method for anelectric power generating apparatus including a thermoelectric element;and an electric power converter which controls electric current when thethermoelectric element generates electric power. The control methodincludes the steps of changing the electric current when thethermoelectric element generates the electric power so as to obtainelectric power values W₁ and W₂ corresponding to at least two differentelectric current values i₁ and i₂, the electric current value i₁ beingsmaller than the electric current value i₂; and estimating an electriccurrent-electric power characteristic based on a combination of theelectric current value i₁ and the obtained electric power value W₁ and acombination of the electric current value i₂ and the obtained electricpower value W₂, obtaining an electric current value at which maximumelectric power can be obtained using the electric current-electric powercharacteristic, and controlling the electric current such that a valueof the electric current becomes equal to the electric current value atwhich the maximum electric power can be obtained when the thermoelectricelement generates the electric power.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description ofpreferred embodiments with reference to the accompanying drawings,wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a schematic diagram showing a configuration of an electricpower generating apparatus according to the invention;

FIG. 2 is a diagram showing a configuration of a thermoelectric elementin FIG. 1;

FIG. 3 is a graph showing an electric current (i)-voltage (V)characteristic, and an electric current (i)-electric power (W)characteristic of a thermoelectric module;

FIG. 4 is a flowchart showing an electric current control process in afirst control mode performed by the electric power generating apparatusin FIG. 1;

FIG. 5 is a graph explaining a method of calculating an optimal electriccurrent value i_(t) in the electric current control process in the firstcontrol mode;

FIG. 6 is a flowchart showing an electric current control process in asecond control mode performed by the electric power generating apparatusin FIG. 1;

FIG. 7 is a graph explaining a method of calculating the optimalelectric current value i_(t) in the electric current control process inthe second control mode;

FIG. 8 is a graph explaining estimation of the characteristic in a casewhere accuracy of a measuring device is low when performing the electriccurrent control process in the second control mode; and

FIG. 9 is a flowchart showing a characteristic discrimination process inthe case shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an exemplary embodiment of the invention will be describedwith reference to accompanying drawings. In order to facilitateunderstanding, components that are the same are denoted by the samereference numerals in the drawings as long as possible, and duplicatedescription will be omitted.

FIG. 1 is a schematic diagram showing a configuration of an electricpower generating apparatus according to the invention. An electric powergenerating apparatus 1 is provided in an exhaust system of anautomobile. The electric power generating apparatus 1 includes athermoelectric module 2 which is formed by connecting multiplethermoelectric elements 20, and an electric power converter 3. As shownin FIG. 2, the thermoelectric element 20 is formed by connectingmultiple p-type semiconductors and n-type semiconductors in a π shape. Ahigh-temperature end side of the thermoelectric module 2 is fixed,through an insulating member 50, to an outer side of an exhaust pipe 4in which exhaust gas flows. A low-temperature end side of thethermoelectric module 2 is connected, through an insulating member 51,to a cooling case 6 in which coolant flows. Both of the insulatingmembers 50, 51 are insulators having good heat conduction. Both of theinsulating members 50, 51 may be made of the same material. However,since the temperature of the insulating member 50 becomes high, at leastthe insulating member 50 needs to have good heat resistance.

An output terminal of the thermoelectric module 2 is electricallyconnected to the electric power converter 3. The electric powerconverter 3 houses an ammeter 30 for measuring an electric current valuei that is the value of electric current flowing in the thermoelectricmodule 2, and a voltmeter 31 for measuring a voltage value V that is thevalue of voltage between output terminals. The electric power converter3 includes a control portion 32 for performing electric current control,therein. The electric power converter 3 has a function of a DC-DCconverter, and is connected to a battery 7. The control portion 32includes a ROM, a RAM, a CPU, and the like. In the exhaust pipe 4, aheat sink 41 that is configured to have a fin shape is provided so thatheat of exhaust gas can be efficiently transmitted to the thermoelectricmodule 2.

The heat sink 41 is heated by exhaust gas flowing in the exhaust pipe 4,and the temperature of the exhaust gas becomes high. A high-temperatureside end surface of each thermoelectric element 20 is heated due to heattransmitted from the heat sink 41 through the insulating member 50.Meanwhile, since heat of a low-temperature side end surface is removedby the coolant flowing in the cooling case 6 due to heat transmissionthrough the insulating member 51. Thus, the temperature of thehigh-temperature side end surface and the temperature of thelow-temperature side end surface become different from each other.Electric current flows in the thermoelectric element 20 due to a Seebeckeffect caused by the temperature difference, and electric power isobtained.

FIG. 3 is a graph showing an electric current (i)-voltage (V)characteristic of the thermoelectric module 2, and an electric current(i)-electric power (W) characteristic of the thermoelectric module 2. InFIG. 3, a solid line shows a case where the temperature of thehigh-temperature side end surface is Th1, and the temperature of thelow-temperature side end surface is Tc1. A dashed line shows a casewhere the temperature of the high-temperature side end surface is Th2,and the temperature of the low-temperature side end surface is Tc2. Therelationship among these temperatures is represented by an equation,(Th1−Tc1)>(Th2>Tc2).

As shown in FIG. 3, the electric current (i)-voltage (V) characteristicof the thermoelectric element varies depending on the temperatures ofboth the end surfaces. When the temperatures are in a given temperaturerange, the electric current (i)-voltage (V) characteristic of thethermoelectric element depends on the temperature difference betweenboth the end surfaces. The relationship between the electric current iand the voltage V is represented by an equation (1) described below whenthe temperatures of both the end surfaces are given temperatures.V=V ₀ −k×i  (1) 0≦i≦V₀/k V=0 i>V₀/k

In the equation (1), V₀ and k are constant values. When the temperatureof the high-temperature side end surface and the temperature of thelow-temperature side end surface are different from each other, thevalue of V₀ and the value of k are different from each other. That is,the values of V₀ and k depend on the temperature of the high-temperatureside end surface and the temperature of the low-temperature side endsurface. Since output electric power W that can be obtained isrepresented by an equation, i×V, the output electric power in the caseof “0≦i≦V₀/k” is represented by an equation (2) described below.W=i×(V ₀ ×k×i)=i×V ₀ −k×i ²  (2)

Since the output electric power W is represented by a quadratic functionwhich is convex upward, the maximum electric power Wmax is obtained atan inflection point. That is, the maximum electric power Wmax isobtained in the case of “dW/di=0”. An electric current value i_(t) atthis time is represented by an equation (3) described below.i _(t) =V ₀/(2×k)  (3)

In the case where the electric power converter 3 controls the electriccurrent flowing in the thermoelectric module 2 such that a value of theelectric current becomes equal to the electric current value i_(t) whenthe electric power is obtained, the electric power can be obtained mostefficiently.

The electric power generating apparatus according to the embodimentcontrols the electric current such that the value of the electriccurrent becomes equal to the electric current value i_(t) when theelectric power is obtained, without using a temperature sensor.

The electric current control will be described specifically. FIG. 4 is aflowchart showing an electric current control process in a first controlmode. FIG. 5 is a graph explaining a method of calculating the electriccurrent value i_(t) in the electric current control process in the firstcontrol mode. The electric current control process is performed by thecontrol portion 32 of the electric power converter 3.

First, in step S1, the value of electric current is changed to anelectric current value i₁ and an electric current value i₂ withreference to the result of measurement performed by the ammeter 30 whenthe electric power is obtained from the thermoelectric module 2. Therelationship between the electric current values i₁ and i₂ isrepresented by an equation, i₁<i₂. That is, the electric current valuei₁ is smaller than the electric current value i₂. For example, it ispreferable that the electric current value i₂ should be substantiallyequal to a value obtained by an equation, 2×i₁. Then, a voltage value V₂corresponding to the electric current value i₁, and a voltage value V₂corresponding to the electric current value i₂ are detected, using thevoltage meter 31.

The electric power converter 3 can instantly change the value ofelectric current supplied to the thermoelectric module 2. Therefore, twocombinations of the electric current value and the voltage value can bedetected in an extremely short time. That is, the speed at which the twocombinations of the electric current value and the voltage value can bedetected is extremely high, as compared to the speed at which thetemperatures of the high-temperature side end surface and thelow-temperature side end surface change. Accordingly, the changes in thetemperatures before and after the electric current value is changed arenegligible.

Next, the optimal electric current value i_(t) is calculated (step S2).When the temperatures of the high-temperature side end surface and thelow-temperature side end surface are the same, the electric current(i)-voltage (V) characteristic can be shown by one straight line. Sincethe combination of the electric current i₁ and the voltage value V₁ (i₁,V₁) and the combination of the electric current i₂ and the voltage valueV₂ (i₂, V₂) are shown by two points on this straight line, the values ofV₀ and k in the equation (1) can be calculated using these two points,as follows.V ₀=(i ₂ V ₁ −i ₁ V ₂)/(i ₂ −i ₁)  (4)k=(V ₁ −V ₂)/(i ₂ −i ₁)  (5)

After the values of V₀ and k are obtained, the optimal electric currentvalue i_(t) can be calculated according to the equation (3).

The optimal electric current value i_(t) may be directly obtained basedon the combinations of the electric current value and the voltage value(i₁, V₁) and (i₂, V₂), according to an equation (6) described below,without calculating the values of V₀ and k separately.i _(t)=(i ₂ V ₁ −i ₁ V ₂)/[2×(V ₁ −V ₂)]  (6)

Also, an electric current value i₃ (=V₀/k) that is an i-axis interceptwhen the voltage value becomes 0 may be obtained, and the optimalelectric current value it may be obtained as a half of the electriccurrent value i₃.

Further, the optimal electric current value i_(t) may be calculated byreading out, from a map stored in the electric power converter 3, theoptimal electric current value i_(t) corresponding to the combinationsof the electric current value and the voltage value, (i₁V₁) and (i₂,V₂).

In each of the aforementioned cases, the obtained optimal electriccurrent value i_(t) matches the optimal electric current value i_(t)represented by the equation (6), and is included in the scope of theinvention.

In step S3, the electric power generation state of the thermoelectricmodule 2 is controlled such that the electric current value becomesequal to the optimal electric current value i_(t), with reference to theoutput of the ammeter 30.

Thus, since thermoelectric power generation can be performed at theoptimal electric current value without using an temperature sensor, heatenergy of the exhaust gas can be efficiently converted to electricenergy, and the electric energy can be recovered.

For example, in the case where the temperatures and the flow amounts ofthe coolant and exhaust gas change, or in the case where a sufficienttime has not elapsed since the engine is started, and therefore thetemperature of the thermoelectric module 2 itself is low and thetemperature of the high-temperature side end surface has not beensufficiently increased, the temperatures of the high-temperature sideend surface and the low-temperature side end surface also change.Therefore, it is preferable that the optimal electric current valuei_(t) should be determined considering a transient change in an engineoperating state.

Accordingly, for example, the process of setting the optimal electriccurrent value i_(t) shown in FIG. 4 is performed at predeterminedtiming. In other words, the process of setting the optimal electriccurrent value i_(t) may be always performed at constant time intervals,or the time intervals at which the process of setting the optimalelectric current value i_(t) is repeatedly performed may be variable.For example, it may be determined whether an engine operating state is asteady state or a transient state based on a signal from an engine ECU(not shown) for controlling the engine, and when the engine operatingstate is the transient state, the time intervals at which the settingprocess is performed may be made short, as compared to when the engineoperating state is the steady state. Alternatively, when the optimalelectric current value i_(t) changes by a large amount as compared tothe value i_(t) that is previously set, the time intervals at which thesetting process is performed may be made short, and when the optimalelectric current value i_(t) changes by a small amount, the timeintervals may be made long.

According to the invention, since the optimal operation of thethermoelectric module can be performed using a simple configuration, theelectric power can be efficiently generated. Also, since it is easy tomaintain a good heat transmission state between the thermoelectricmodule and the heat source (i.e., exhaust gas and the coolant), theelectric power generation efficiency can be improved. Also, an existingelectric power generating apparatus can be performed with higherefficiency by providing the electric power converter 3.

Next, an electric current control process in a second control mode willbe described. FIG. 6 is a flowchart showing the electric current controlprocess in the second control mode. FIG. 7 is a graph explaining amethod of calculating the optimal electric current value i_(t) in thiselectric current control process in the second control mode. Thiselectric current control process is performed by the control portion 32of the electric power converter 3, as in the case of the electriccurrent control process in the first control mode.

First, in step S11, the value of the electric current is changed to theelectric current value i₁ and the electric current value i₂ withreference to the result of measurement performed by the ammeter 30 whenthe electric power is obtained from the thermoelectric module 2. Therelationship between the electric current values i₁ and i₂ isrepresented by an equation, i₁<i₂. That is, the electric current valuei₁ is smaller than the electric current value i₂. For example, it ispreferable that the relationship between the electric current values i₁and i₂ should be represented by an equation, i₂≦2×i₁. Then, the voltagevalue V₁ corresponding to the electric current value i₁, and the voltagevalue V₂ corresponding to the electric current value i₂ are detected,using the voltage meter 31.

Next, an electric power amount W₁ in the case where the electric currentvalue is i₁ and the voltage value is V₁, and an electric power amount W₂in the case where the electric current value is i₂ and the voltage valueis V₂ are calculated (step S12). The electric power amount W₁ isrepresented by an equation, W₁=i₁×V₁. The electric power amount W₂ isrepresented by an equation, W₂=i₂×V₂.

Then, a calculation is performed to obtain an electric current value(i)-electric power (W) characteristic curve which passes through threepoints (0, 0), (i₁, W₁), and (i₂, W₂). The characteristic curve isrepresented by the equation (2). The values of V₀ and k can be obtainedaccording to equations (7) and (8) described below.V ₀=(i ₂ ² W ₁ −i ₁ ² W ₂)/{i ₁ ×i ₂×(i ₂ −i ₁)}  (7)k=(i ₂ ×W ₁ −i ₁ ×W ₂)/{i ₁ ×i ₂×(i ₂ −i ₁)}  (8)

After the values of V₀ and k are obtained, the optimal electric currentvalue i_(t) can be obtained according to the equation (3) (step S14).

The optimal electric current value i_(t) may be calculated by readingout, from a map stored in the electric power converter 3, the values ofV₀ and k corresponding to the combinations of the electric current valueand the electric power value (i₁, W₁) and (i₂, W₂), instead ofcalculating the values of V₀ and k using the equations (7) and (8).

In step S15, the electric power generation state of the thermoelectricmodule 2 is controlled such that the electric current value becomesequal to the optimal electric current value i_(t), with reference to theoutput of the ammeter 30.

In this case, the electric power amount is obtained based on theelectric current value and the voltage value. However, an electric powervalue may be obtained by providing a wattmeter instead of the voltmeter31 shown in FIG. 1. Also, the electric power value may be obtained basedon the amount of electric power supplied to the battery 7.

Meanwhile, each of the output values of the ammeter, the voltmeter, andthe wattmeter has an error. When the accuracy of each measuring deviceis relatively low, there may be plural electric current (i)-electricpower (W) characteristic lines which pass through the two points (i₁,W₁) and (i₂, W₂), as shown in FIG. 8, considering the error.Hereinafter, description will be made of an electric current controlprocess by which the optimal electric current value i_(t) can beobtained with high accuracy even when using a measuring device havingrelatively low accuracy. FIG. 9 is a flowchart showing part of thiscontrol process. This control process is inserted between step S13 andstep S14 of the control process shown in FIG. 6.

In step S13, characteristic lines A and B shown in FIG. 8 are decided ascandidates for the electric current (i)-electric power (W)characteristic line. Further, in step S21, the electric current value ischanged to a value obtained by 2×i₂, with reference to the result ofmeasurement performed by the ammeter 30. The voltage value V3 at thistime is obtained.

In step S22, the voltage value V3 and 0 are compared to each other. Whenthe voltage value V3 is not 0, the characteristic line B is selected(step S23). When the voltage value V3 is 0, the characteristic line A isselected (step S22). Thus, the optimal electric current value i_(t) canbe obtained with high accuracy.

In this case, the voltage value corresponding to the electric currentvalue 2×i₂ is additionally obtained. However, when the wattmeter isprovided, the electric power value corresponding to the electric currentvalue 2×i₂ may be obtained. Also, the electric current value when thevoltage value or the electric power value is additionally obtained isnot limited to the value obtained by 2×i₂. One of the two electriccurrent values may be made a candidate for the optimal electric currentvalue, or an electric current value k×i₂ (it is preferable that thevalue of k should be larger than 1, and should be equal to or smallerthan 2) may be used.

In these control modes as well, the same effects as in the first controlmode can be obtained.

Description has been made of a thermoelectric power generating apparatusprovided in an exhaust system of an automobile. However, the inventionis not limited to this. The invention can be applied to, for example, athermoelectric power generating apparatus which converts heat energy ofcombustion gas of various types of internal combustion engines orcombustion devices, or heat energy of drain warm water to electricenergy and recovers the electric energy.

1. An electric power generating apparatus comprising: a thermoelectricelement; and an electric power converter which controls electric currentwhen the thermoelectric element generates electric power, wherein theelectric power converter changes the electric current when thethermoelectric element generates the electric power so as to obtainvoltage values V_(1 and V) ₂ corresponding to at least two differentelectric current values i₁ and i₂, the electric current value i₁ beingsmaller than the electric current value i₂; and the electric powerconverter controls the electric current such that a value of theelectric current becomes equal to an electric current value i_(t) whenthe thermoelectric element generates the electric power, the electriccurrent value i_(t) satisfying an equation, i_(t)=(i₂V₁−i₁V₂)/2(V₁−V₂).2. The electric power generating apparatus according to claim 1, whereinthe electric power converter includes an ammeter that measures a valueof electric current flowing in the thermoelectric element, and avoltmeter that measures a value of voltage between output terminals ofthe thermoelectric element.
 3. The electric power generating apparatusaccording to claim 1, wherein the thermoelectric element includes ahigh-temperature side end-surface and a low-temperature side endsurface, and generates the electric power according to a temperaturedifference between the high-temperature side end surface and thelow-temperature side end surface.
 4. The electric power generatingapparatus according to claim 3, wherein the high-temperature side endsurface is provided in a vicinity of an exhaust passage of anautomobile, and a cooling device is provided on the low-temperature sideend surface.
 5. An electric power generating apparatus comprising: athermoelectric element; and an electric power converter which controlselectric current when the thermoelectric element generates electricpower, wherein the electric power converter changes the electric currentwhen the thermoelectric element generates the electric power so as toobtain electric power values W₁ and W₂ corresponding to at least twodifferent electric current values i₁ and i₂, the electric current valuei₁ being smaller than the electric current value i₂; the electric powerconverter estimates an electric current-electric power characteristicbased on a combination of the electric current value i₁ and the obtainedelectric power value W₁ and a combination of the electric current valuei₂ and the obtained electric power value W₂; the electric powerconverter obtains an electric current value at which maximum electricpower can be obtained using the electric current-electric powercharacteristic; and the electric power converter controls the electriccurrent such that a value of the electric current becomes equal to theelectric current value at which the maximum electric power can beobtained when the thermoelectric element generates the electric power.6. The electric power generating apparatus according to claim 5, whereinwhen plural electric current-electric power characteristics areestimated based on the combination of the electric current value i₁ andthe obtained electric power value W₁ and the combination of the electriccurrent value i₂ and the obtained electric power value W₂, the electricpower converter obtains one of a voltage value and an electric powervalue corresponding to an electric current value i₃, the electriccurrent value i₃ being different from the electric current values i₁ andi₂, and the electric power converter selects the electriccurrent-electric power characteristic from among the estimated pluralelectric current-electric power characteristics based on a combinationof the electric current value i₃ and one of the voltage value and theelectric power value corresponding to the electric current value i₃. 7.The electric power generating apparatus according to claim 5, whereinthe electric power converter includes an ammeter that measures a valueof electric current flowing in the thermoelectric element, and avoltmeter that measures a value of voltage between output terminals ofthe thermoelectric element.
 8. The electric power generating apparatusaccording to claim 5, wherein the electric power converter includes anammeter that measures a value of electric current flowing in thethermoelectric element, and a wattmeter that measures a value ofelectric power output from the thermoelectric element.
 9. The electricpower generating apparatus according to claim 5, wherein thethermoelectric element includes a high-temperature side end surface anda low-temperature side end surface, and generates the electric poweraccording to a temperature difference between the high-temperature sideend surface and the low-temperature side end surface.
 10. The electricpower generating apparatus according to claim 9, wherein thehigh-temperature side end surface is provided in a vicinity of anexhaust passage of an automobile, and a cooling device is provided onthe low-temperature side end surface.
 11. A control method for anelectric power generating apparatus including a thermoelectric element;and an electric power converter which controls electric current when thethermoelectric element generates electric power, comprising the stepsof: changing the electric current when the thermoelectric elementgenerates the electric power so as to obtain voltage values V₁ and V₂corresponding to at least two different electric current values i₁ andi₂, the electric current value i₁ being smaller than the electriccurrent value i₂; and controlling the electric current such that a valueof the electric current becomes equal to an electric current value i_(t)when the thermoelectric element generates the electric power, theelectric current value i_(t) satisfying an equation,i_(t)=(i₂V₁−i₁V₂)/2(V₁−V₂).
 12. A control method for an electric powergenerating apparatus including a thermoelectric element; and an electricpower converter which controls electric current when the thermoelectricelement generates electric power, comprising the steps of: changing theelectric current when the thermoelectric element generates the electricpower so as to obtain electric power values W₁ and W₂ corresponding toat least two different electric current values i₁ and i₂, the electriccurrent value i₁ being smaller than the electric current value i₂; andestimating an electric current-electric power characteristic based on acombination of the electric current value i₂ and the obtained electricpower value W₁ and a combination of the electric current value i₂ andthe obtained electric power value W₂, obtaining an electric currentvalue at which maximum electric power can be obtained using the electriccurrent-electric power characteristic, and controlling the electriccurrent such that a value of the electric current becomes equal to theelectric current value at which the maximum electric power can beobtained when the thermoelectric element generates the electric power.